This invention relates generally to photovoltaic devices and to methods for their manufacture. More specifically, the invention relates to lightweight, high efficiency photovoltaic devices having a photovoltaically active area which is free of opaque current collecting grids.
Photovoltaic devices provide a self-contained, reliable source of electrical power. Initially, photovoltaic devices were based on single crystal semiconductor materials; and consequently, they were expensive, fragile, restricted in size, and relatively heavy. Subsequently, techniques were developed for the preparation of large area, thin film semiconductor materials having very good electrical properties. These materials can be employed to fabricate photovoltaic devices which are light in weight, large in area, low in cost and of high efficiencies.
Because of these advantageous properties, thin film photovoltaic devices have found a number of uses, both in terrestrial and aerospace applications. A typical thin film photovoltaic device is fabricated upon a relatively lightweight substrate, and includes a body of photovoltaically active semiconductor material interposed between a top and a bottom electrode, and at least one of these electrodes is fabricated from a transparent, electrically conductive material. Most preferably, the photovoltaic devices are configured as modules of individual cells, supported on a common substrate, and connected in a series, parallel or mixed series parallel electrical relationship. In many applications, and in aerospace applications in particular, weight is at a premium, and thin film photovoltaic devices are attractive since they can deliver large amounts of power, on a weight basis, as compared to other sources of electrical energy.
A number of parameters can be optimized to maximize the power output of photovoltaic modules. The first involves the optimization of the photovoltaic material itself. Improvements in the quality and stability of photovoltaic alloys will improve the efficiency of devices fabricated therefrom. In addition, optimization of device configuration, such as for the use of stacked tandem, spectrum splitting cells, has further increases device efficiency. Another approach to increasing overall device efficiency involves lowering the resistive losses in the device. As noted above, photovoltaic devices require a transparent, electrically conductive electrode on their light incident side. These electrodes are generally fabricated from conductive metal oxides such as indium tin oxide and the like; and while they are electrically conductive, their conductivity is much lower than that of metals, and they can be significant sources of resistive loss. As a consequence, it is conventional in the art to include current collecting grid structures on the light incident surface of photovoltaic devices. These grids are fabricated from a high conductivity metallic material and serve to shorten the path of carriers through the transparent conductive electrode layer. U.S. Pat. No. 5,131,954 discloses a large-area photovoltaic array comprised of a plurality of interconnected devices, in which each device includes a current collecting grid.
While inclusion of grid lines does significantly decrease the resistance of a photovoltaic device, the grid lines are opaque and decrease the photovoltaically active area of a device. Current collecting grids can be eliminated if the dimensions of a photovoltaic device are kept quite small, typically such that current paths through the electrode are less than 0.25 cm; but then, current output of the device is very low, and if practical current levels are to be achieved, a large number of such small devices must be interconnected thereby giving rise to resistive losses and a decrease in active surface area of a module resultant from the many interconnections. As a consequence, in the design of photovoltaic modules, the dimensions of the current collecting grid, and hence resistive loss, is carefully balanced against loss of active area.
It is also known in the prior art that the overall conversion efficiency of a photovoltaic device can be increased by maximizing the amount of light absorbed by the photovoltaic device. This is accomplished by including back reflector structures in the device to redirect unabsorbed light back through photovoltaically active layers, and also by incorporating anti-reflective coatings on the light incident side of the photovoltaic device. As is known in the art, an anti-reflective condition can be established by including an anti-reflective layer in the photovoltaic device. The thickness and refractive index of the anti-reflective layer are selected so that an anti-reflective condition is established for wavelengths corresponding to the maximum illumination and/or maximum sensitivity of the photovoltaic device.
The anti-reflective layer may be separate from the transparent, conductive electrode; however, in many instances, the top, transparent conductive electrode layer itself is employed as an anti-reflective layer. In either case, the thickness of the   T  =      λ          4      ⁢              xe2x80x83            ⁢      n      
anti-reflective layer is determined by the formula:
wherein T is the thickness of the anti-reflective layer, xcex is the wavelength being anti-reflected and n is the index of refraction of the anti-reflective layer. In those instances where the transparent conductive oxide materials are used as the top electrode and anti-reflective layer, n will be approximately 2. These layers are known in the art as one-quarter xcex anti-reflective layers, and have heretofore been preferred because they provide broad band anti-reflection. In most photovoltaic devices, the anti-reflective conditions are preferably established for wavelengths in the range of 450 to 600 nanometers, and accordingly, conventionally employed anti-reflective layers are usually in the thickness range of 60 to 75 nanometers. Reflection enhancing and suppressing layers for photovoltaic devices are shown in the prior art, for example in U.S. Pat. Nos. 5,569,332 and 4,419,533, and U.S. Pat. No. 4,471,155 specifically shows a photovoltaic device having a one-quarter xcex anti-reflection layer which is used in combination with a current collecting grid.
One aspect of the present invention resides in the recognition of the fact that thicker anti-reflective layers corresponding to a three-quarters xcex condition are actually preferable in photovoltaic devices. The thicker layers provide a narrower band anti-reflective condition than do quarter xcex layers, and their use appears to be contraindicated; however, the thicker layers have a significantly higher electrical conductivity, and when coupled with appropriate module design, allow for elimination of current collecting grids, thereby increasing the photovoltaically active area of the devices.
As will be described in greater detail hereinbelow, the present invention is directed to photovoltaic devices having enhanced efficiencies at preselected wavelengths, which devices are of gridless construction. The devices of the present invention are highly efficient, simple to fabricate and do not require current collecting grid structures which decrease active area of the device, necessitate additional processing steps, and can be sources of defects. These and other advantages of the present invention will be apparent from the drawings, discussion, description and examples which follows.
There is disclosed herein a monolithic photovoltaic module which has an enhanced absorption of light at a preselected wavelength xcex. The module is fabricated upon an electrically insulating substrate which has a plurality of photovoltaic devices disposed thereupon and electrically connected in series. Each photovoltaic device includes a bottom electrode supported on the substrate, a body of photovoltaic material disposed on the bottom electrode, and a transparent, electrically conductive top electrode disposed upon the body of photovoltaic material and separated from the bottom electrode thereby. In accord with the present invention, the thickness of the top electrode is given by the formula   T  =            3      ⁢              xe2x80x83            ⁢      λ              4      ⁢      n      
wherein xcex is a wavelength of light in the range of 450-600 nm, and n is the index of refraction of the top electrode. The module further includes means for establishing a series electrical connection between members of the plurality of photovoltaic devices, and a first and a second terminal in electrical communication with a plurality of photovoltaic devices for withdrawing electrical power therefrom.
In particular embodiments, the top electrode has a thickness in the range of 170 to 225 nm. In certain embodiments, the electrical communication between the individual photovoltaic devices is established by high conductivity shunt paths established therethrough, and extending between the top electrode of a given photovoltaic device, and the bottom electrode of an adjoining device. In certain embodiments, the body of photovoltaic material comprises a plurality of layers of hydrogenated, group IV semiconductor alloy material configured as stacked, tandem, p-i-n type devices.
In accord with the present invention, there are also provided methods for fabricating the modules and devices thereof.